Just to be clear.
You modify your Steam shortcut by clicking on Properties and you modify the Target line so you have this (it will look different depending on where you have steam installed, of course):
"E:\Program Files\Valve\Steam\Steam.exe" -beta roclient50
You click on that shortcut and Steam opens and should automatically download the beta update. This is where things get a little fuzzy. You may be able to play now, or you may need to "Verify Cache" and download it again. I had problems here, but eventually it did work. Just let it finish downloading whatever it wants.
Once the downloading is sorted out, you click on Red Orchestra in the My Games tab in Steam.
It should start and show the new RED menu screen. If you see the old GRAY menu then you are not running the beta. The error you mentioned, where the server refuses to download the map, happens when you try to play on a beta server when you are not running the beta yourself. Something must have gotten mixed up and reverted back to the non-beta version. You shouldn't have to download any maps.
This is what you want to see: